1. Field of the Invention
Fabrication of silica glass articles by the colloidal sol-gel process.
2. Description of Related Art
After many years of effort, the colloidal sol-gel process for making silica glass is coming into prominence.
The first step in the basic process is preparation of a dispersion (or xe2x80x9csolxe2x80x9d) of fine silica particles in water by shear-mixing. To avoid premature gelling, a xe2x80x9cstabilizerxe2x80x9d is usually added to the prepared dispersion. The stabilizerxe2x80x94commonly a tetrammonium hydroxidexe2x80x94maintains the sol alkaline, so that the particles are charged negatively and repel each other. Gelation is begun by use of a pH-lowering gelling agentxe2x80x94e.g., methyl formate or another water-soluble hydrolyzable esterxe2x80x94which effectively neutralizes charge on the particle surfaces thereby permitting particle agglomeration, and initiating gelling. Thereafter, the gelatinous medium is dried and consolidated to make pore-free glass. The process would supplant high-temperature vapor-transport procedures now in commercial use in the manufacture of silica glass.
Preparation of silica using the sol-gel process has been thwarted by a number of problems. A meaningful breakthrough is described in U.S. Pat. No. 5,240,488, Aug. 31, 1993. That patented process overcomes the well-recognized yield problem for kilogram and larger crack-free bodies. It depends on a small inclusion of an organic polymeric material, which after serving its purpose decomposes and/or volatilizes during subsequent high-temperature processing. A near-term benefit of the process is to permit substitution of sol-gel-derived overcladding tubes for the commercial (soot-derived) tubing now used in the fabrication of composite preforms from which optical fiber is produced.
The individual steps have been restudied to optimize the process.
While dispersions had earlier been prepared from ground sand, fumed silica with its high purity and better control has become the more usual starting material for fiber and other demanding purposes. U.S. Pat. No. 5,116,535, May 26, 1992, best represents the state of the art for preparation of fumed silica dispersions.
Consistent with earlier practice, the patented procedure relies on shear-mixing of a particle-water mixture, however, of higher silica loading than desired in the ultimate product. Following mixing, the initial suspension is diluted with additional water to yield the final suspension of desired silica loading, typically of xe2x89xa735 wt. % silica. Stabilizer is avoided before and during mixingxe2x80x94in accordance with current practice is thereafter introduced in desired amount for required shelf-life.
U.S. Pat. No. 5,246,624, Sept. 21, 1993, describes and claims a different method for forming a silica-water dispersion. That method requires acidification of the initial mixture prior to dispersing for the stated purpose of enhancing wetting of the silica particles and thereby aiding mixing. Such acidified dispersions are undesirable for many purposesxe2x80x94may contain undesirable contaminants; may cause corrosion of dispersion apparatus; upon addition of stabilizer may result in salt formation which modifies the rheological properties of the dispersion.
Examples in ""624 introduce excess silica before dispersing, and provide for dilution afterwards as in ""535. Conventional pH-increase stabilizes the dispersed material.
Silica-water dispersions may serve uses unrelated to sol-gel fabrication. For example, they are used for Chemical Mechanical Polishing (CMP) in the fabrication of integrated circuits.
A dispersing process alternative to that of U.S. Pat 5,116,535 has been used successfully in sol-gel manufacture, and is applicable to production of overcladding tubes for optical fiber. Its evolution is traced to an improved fundamental understanding of the colloidal sol-gel process.
In accordance with the invention, dispersing efficacy is assured by adjusting pH of the silica-water mixture prior to or during dispersing. The amount of pH-adjusting agent added results in an increase of the particle-to-particle attractive forcesxe2x80x94the same forces responsible for initiation of gelation. In fact, adjustment increases gelation ratexe2x80x94destabilizes the initial mixturexe2x80x94so that dispersing efficiency is, in this sense, found related to the phenomenon of gelation itself. It is generally anticipated that the initial mixture, in being constituted of fumed silica particles in water, is at sufficiently low pH so that adjustment in accordance with the teaching requires increasing pH.
By increasing pH so as to fall within this region of increased rate of gelation, the xe2x80x9csteady-statexe2x80x9d viscosity of the initial mixture, as measured during or immediately after dispersing, is increased. The invention teaches a direct relationship between this viscosity and effectiveness of dispersing. A xe2x80x9cfinal viscosityxe2x80x9d value, measured at a reference pH value suitable for stabilization of the dispersion, is found to vary in a direction opposite to that of the steady-state viscosity, so that dispersing effectiveness may be measured in terms of this xe2x80x9cfinal valuexe2x80x9d.
As a consequence of the present work, it is realized that one role played by the excess silica loading of U.S. Pat. No. 5,116,535, is to increase viscosityxe2x80x94with the effect of improving dispersing efficiency. An embodiment of this invention employs excess loading as an ancillary procedure, under circumstances that pH adjustment and excess loading cooperate to produce a further increased steady-state value of viscosity.
Terminology
Primary Particlexe2x80x94Void-free silica particle in which all molecule-to-molecule association is due to siloxane bonding. (Primary particles are sometimes described in the literature as xe2x80x9cspherical particlesxe2x80x9d.)
Aggregatexe2x80x94Mass of two or more primary particles held together by siloxane bonding.
Agglomeratexe2x80x94Mass of primary particles and/or aggregates held together by means other than siloxane bonds, e.g., by van der Waals forces or hydrogen bonds.
Silica Sol or Silica Dispersionxe2x80x94Silica-water suspension in which the suspended medium consists of silica primary particles or aggregatesxe2x80x94in which agglomerates are substantially excluded.
Steady-State Viscosityxe2x80x94Viscosity attained during dispersingxe2x80x94so named since viscosity, while dropping substantially at the commencement of mixing, changes relatively little from commencement to the completion of dispersing. (In the work reported here, the value was measured immediately after mixing.)
Final Viscosityxe2x80x94Viscosity of a stabilized solxe2x80x94used as an indicator of dispersing efficacy and of attainment of a sol of substantially unagglomerated silica. Final viscosity in the reported work, is measured at pH 12.8 and with a silica loading of 44 wt. %. The terminology does not impose a requirement for any specific loading nor for stabilization at this or any other pH on the claimed inventionxe2x80x94use of an unstabilized dispersion is contemplated.
Initial Mixturexe2x80x94As differentiated from xe2x80x9cdispersionxe2x80x9d, this is the initial undispersed silica-water mixture as may be introduced into the mixing apparatus. (Under some circumstances, addition of silica may be stepwise or continuous during dispersing, so that the xe2x80x9cinitial mixturexe2x80x9d may not exist in totality at any instant.)
Gelation Regionxe2x80x94Expressed in terms of pH range, this defines the initial mixture, now modified by addition of alkaline additive, including a peak at which gelation proceeds at a rate an order of magnitude larger than the boundary values of the range. (Under conditions of a preferred embodiment, in which the initial mixture consists largely of an unmodified aqueous mixture of fumed silica, the peak pH value lies at about pH7.5 and the range extends from pH6 to pH9. Outside the boundary values a period of several days is required for xe2x80x9cgelationxe2x80x9dxe2x80x94whereas at the peak, here identified as corresponding with an increase in viscosity to xe2x89xa750,000cp, a period of 24 hours suffices.)